This invention relates to copolyesters of p-hydroxybenzoic acid, isophthalic acid, hydroquinone and 2,2-bis(4-hydroxyphenyl)propane.
The use of objects molded from synthetic polymers has expanded rapidly in the last several decades. In particular, polyesters have widely gained acceptance for general molding applications. Although most polyesters have mechanical properties suitable for general molding applications, most polyesters are not suitable for high strength service because the mechanical properties are not sufficiently high. One group of polyesters that is suitable for high strength service without the need of a reinforcing agent is a new class of polymers exhibiting a general overall balance of mechanical properties substantially enhanced over previous polymers. These polymers have been described by various terms, including "liquid crystalline", "liquid crystal", and "anisotropic". Briefly, the polymers of this new class are thought to involve a parallel ordering of the molecular chains. The state wherein the molecules are so ordered is often referred to either as the liquid crystal state or the nematic phase of the liquid crystal state. These polymers are prepared from monomers which are generally long, flat and fairly rigid along the long axis of the molecule and have chain extending linkages that are either coaxial or parallel. For example, liquid crystal copolyesters have been prepared from the following fairly rigid molecular species: p-hydroxybenzoic acid, hydroquinone and isophthalic acid. However when ratios of the monomers are selected to provide tractable polymers, the glass transition temperature is generally low and the high temperature mechanical properties are reduced.
Applicant has now found that tractable copolyesters of enhanced glass transition temperature can be prepared from p-hydroxybenzoic acid, isophthalic acid, hydroquinone and a surprisingly small ratio of 2,2-bis(4-hydroxyphenyl)propane and that the enhancement in glass transition temperature is achieved without sacrifice in liquid crystallinity. Thus the copolyesters of the present invention are capable of forming a thermotropic melt phase at a temperature below about 340.degree. C., have an inherent viscosity of at leat 0.3, and a glass transition temperature of at least 160.degree. C. and consist essentially of the following divalent radicals: ##STR1## wherein the range of radical A is from 10 to 40 mole percent of the copolyester and the range of radical C is from 82 to 95 mole percent of the total of radicals C and D.
The copolyesters of the present invention are prepared by an acidolysis procedure in which esters of monocarboxylic acids such as acetic acid are prepared by esterification of the phenolic hydroxy groups of p-hydroxybenzoic acid, hydroquinone and 2,2-bis-(4-hydroxyphenyl)propane and the esters are reacted with isophthalic acid at a temperature in the range of about 250.degree. to about 340.degree. C. The esters can be prepared in situ by reaction of the phenols with acetic anhydride.
In general the copolyester should have an inherent viscosity of at least about 0.3 and more preferably at least about 0.5 in order to provide adequate mechanical properties. The inherent viscosity can, if desired, be increased to values of 0.7 or even higher using conventional techniques such as melt or solid state polymerization. The inherent viscosity is determined at 30.degree. C. with a solution of 0.5 gram of copolyester per 100 ml of a solvent composed of 60 parts by weight of phenol and 40 parts by weight of tetrachloroethane.
The liquid crystallinity of the copolyesters can be demonstrated from the optical anisotropy of the melt above the flow temperature and from the melt viscosity behavior versus rate of shear. The degree of ordering in the melt produces a thixotropic effect and the melt viscosity can reach extremely high values at low rates of shear depending on the degree of ordering of the polymer molecules in the melt.
Advantageously the ratios of monomers in the copolyesters of the present invention can be selected to provide copolyesters which are tractable at a low temperature, the flow temperature at which they form a thermotropic melt phase being substantially below 340.degree. C. and to have a high long-term use temperature associated with the high glass transition temperature. Preferably the glass transition temperature is at least about 160.degree. C. Among the preferred compositions, are copolyesters containing from about 20 to about 35 mole percent p-hydroxybenzoic acid units, and copolyesters containing from about 5 to about 12 mole percent of 2,2-bis(4-hydroxyphenyl)propane based on the total of hydroquinone and 2,2-bis(4-hydroxyphenyl)propane.
The copolyesters of the present invention can contain nucleating agents, fillers, pigments, glass fibers, mineral reinforcing agents, antioxidants, stabilizers, plasticizers, lubricants, fire-retardants and other additives.
The copolyesters are useful for preparing molded objects, films, fibers and the like. For example, they may be injection molded by conventional means and they may be formed into fibers by melt spinning and subsequently drawn, and further processed according to conventional techniques.